Heat shrinkable polymeric films are widely used for packaging of fresh and processed meats, cheeses, poultry and a large number of non-food items. Some of the films are formed into heat shrinkable bags or pouches which are supplied to the meat packer.
Processing and distribution requirements demand that the film and bags made therefrom have a wide range of difficult-to-meet properties. Of great importance to the packer is the capability of a bag to survive physically the process of being filled, evacuated, sealed, heat shrunk, and shipped to various distribution points. The bag must be strong enough to survive the material handling and also physically protect the enclosed product.
It is also desirable that the bag serve as a barrier to oxygen and other materials which could cause spoilage of a product wrapped in the film. Furthermore, it is desirable that such a bag be heat shrinkable below the boiling point of water so that high temperatures do not have to be employed to shrink the bag and a convenient heat transfer medium such as hot water can be employed.
One successful heat shrinkable bag having a barrier layer of saran is disclosed in U.S. Pat. No. 3,741,253 which issued to H. J. Brax et al on June 26, 1973. In the Brax patent the multi-ply film has a substrate or sealing layer of cross-linked EVA, a central layer of saran, and an outer or abuse layer of EVA. The inner or sealing layer is referred to as the base or substrate layer. In this instance it is extruded as a tube and is cross-linked and the saran and abuse layer of EVA are extrusion coated thereonto. Thus, a tube is formed which is thereafter heated to orientation temperature, stretched and then rapidly cooled to form an oriented or heat shrinkable film. Bags are then made by transversely sealing across the tubular structure. Hence, the substrate layer is also the inner layer which is heat sealed to itself to form the seal across the bottom of a bag. The other EVA layer protects the saran from scratches and pin holes and receives the abuse of handling and contact with other packages and surfaces. Hence, it known as the "abuse" layer.
As linear copolymers of ethylene known as LLDPE began to become commercially available, they were incorporated into various monolayer and multi-layer structures. In order that the LLDPE layers would extrude more readily and adhere to adjacent film layers, it was found convenient to blend the LLDPE with EVA. Typical patents which disclose layers of LLDPE blended with EVA in a multi-layer, barrier structure are U.S. Pat. No. 3,682,767 which issued on Aug. 8, 1972 to Arthur Wray Britton et al; U.S. Pat. No. 4,456,646 which issued to Nishimoto et al on June 26, 1984; U.S. Pat. No. 4,495,249 which issued on Jan. 22, 1985 to Ohya et al; U.S. Pat. No. 4,424, 243 which issued on Jan. 3, 1984 to Nishimoto et al; and, U.S. Pat. No. 4,457,960 which issued on July 3, 1984 to David L. Newsome.
One of the primary objects of the present invention is to provide a multi-layer barrier film employing layers of LLDPE in which the adhesion to adjacent saran layers is improved over similar structures in the prior art. In the Britton patent, mentioned above, a copolymer of ethylene with ethylacrylate is mentioned along with the copolymer of ethylene/vinyl-acetate. But no specific example showing the use of ethylene/ethyl-acrylate is given. In Nishimoto '243, mentioned above, the broad term ethylene/alkyl-acrylate is used. But, the only example is, again, ethylene/ethyl-acrylate with a crystalline melting point of 89.degree. C. wherein 18% of the copolymer is ethyl-acrylate. In Nishimoto '243 the ethylene/ethyl-acrylate is used only as an unblended adhesive layer. In Ohya '249 the same ethylene/ethyl-acrylate copolymer is used having a melt index of 6 and crystalline melting point of 90.degree. C. and 18% by weight of ethyl-acrylate unit. In Ohya '249 the ethylene/ethyl-acrylate copolymer is blended with polyethylene modified by carboxylic acid and used as an adhesive layer.
It is another primary object to employ an ethylene copolymer in a blend with LLDPE or VLDPE to achieve a superior barrier film.
These and other objects are achieved by the invention which is summarized below.